|Year|Album|Song|Duration|Artist|
|---|---|---|---|---|
|2010|TRAX Mini Album Volume Up|"Healing"||with TRAX|
|2010|Oh!|"Boys & Girls"||with Girls' Generation|
|2010|1st Asia Tour "Into The New World"|"Barbie Girl"||Duet with Jessica|
|2012|History of Salary Man OST|"Bravo"|03:12|Duet with Leeteuk|
|2012|One Dream (Single)|"One Dream"|02:29|with BoA & Henry|
|2012|MAMA|"Two Moons"|02:59|with EXO|
|2012|Only One|"One Dream"|02:29|with BoA & Henry|
|2014|Toheart|"Delicious"||Duet with Woohyun from Infinite|

Question: how many years were between the albums only one and toheart?
Answer: 2